A transistor formed over a glass substrate or the like is manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in a liquid crystal display device. Although a transistor including amorphous silicon has low field effect mobility, it can be formed over a larger glass substrate. On the other hand, although a transistor including polycrystalline silicon has high field effect mobility, it is not suitable for being formed over a larger glass substrate.
In contrast to a transistor including silicon, attention has been drawn to a technique by which a transistor is manufactured using an oxide semiconductor and is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and is used as a switching element of a pixel or the like of a display device.